The present invention relates generally to current transformers, and in particular to in-service testing of current transformers used in power generation and distribution systems.
Power generation and distribution systems use current transformers at numerous locations throughout the system as the primary sensor to monitor the current flowing through critical assemblies. The output of the current transformer provides a representation of the current flowing through the assembly that is being monitored. Associated monitoring and control instrumentation in combination with the current transformer may provide critical system functions such as overload protection and power usage monitoring. The importance to plant operation and reliability of current transformers cannot be over-emphasized. Problems with current transformers and associated monitoring and control instrumentation may cause very expensive and costly outages that are usually avoided at all costs. Therefore, it is imperative that these units be reliable and perform as designed at all times, and that the operating condition of a current transformer be known throughout its operating life.
However, using conventional techniques for evaluating the condition of a current transformer requires extensive downtime and associated expense. To evaluate a current transformer using conventional techniques, the primary conductor of the transformer must be disconnected from the assembly it is monitoring so that current flow through the primary is interrupted. Then, after the primary current is interrupted, a test current in accordance with IEEE specifications (IEEE C57.13 (1993) and ANSI C57.13.1 (1981)) is used to construct an excitation curve that reflects the operating characteristics of the current transformer. The excitation curve is then evaluated to determine if there are problems with the current transformer.
Since disconnecting the current transformer is expensive and requires extensive downtime, testing is rarely performed except when problems occur. However, waiting for a problem to occur results in unexpected outages which are very expensive in terms of lost generation capacity, labor, and materials, and of course substantial inconvenience to customers.
The present current transformer test system provides a method of testing a current transformer that has a primary winding and a secondary winding. An operating current continues to flow through the primary winding during testing of the current transformer. A controllable load is applied to the current transformer secondary winding. The controllable load is varied over a range of load settings including a maximum current setting and a maximum voltage setting. At a plurality of load settings within the range of load settings, a current flowing through the current transformer secondary winding is measured. Also, a voltage across the current transformer secondary winding is measured. An actual excitation curve is generated from the measured currents and voltages corresponding to the plurality of load settings.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.